Such a hybrid drive-train of a motor vehicle comprises an internal combustion engine with a driveshaft, an adjustable-ratio transmission with a transmission input shaft and a transmission output shaft in driving connection with an axle transmission or transfer box, an electric machine that can be operated at least as a motor having a rotor that is drive-connected to the transmission input shaft, and a separator clutch in the form of an automated friction clutch arranged between the driveshaft of the internal combustion engine and the transmission input shaft. With such a hybrid drive-train, starting from electrically powered driving operation with the separator clutch open, when increased power is called for the previously started internal combustion engine is accelerated to the rotational speed of the transmission input shaft and then coupled to the transmission input shaft by closing the separator clutch.
A parallel-action hybrid drive-train of the above type is widely known and can be regarded as the standard design of a parallel hybrid drive-train. It has the advantage that the motor vehicle concerned can optionally be driven in purely electrically powered operation with the internal combustion engine switched off, in purely internal combustion engine powered operation with the electric machine power switched off, or in hybrid driving operation with combined drive power from the internal combustion engine and the electric machine. During internal combustion engine driving operation the electric machine can if necessary also be operated as a generator and the current so produced can be used for supplying an on-board electrical system and/or for charging an electrical energy accumulator. In general the electric machine can also be operated as a generator when the motor vehicle is braked and the kinetic energy recovered thereby can be stored in an electrical energy accumulator.
The transmission is preferably designed to be automatically controlled, such that the gear ratio effective between the input and output shafts can be adjusted either in steps or continuously. Thus, the transmission can be an automated variable-speed transmission that can be shifted with traction force interruption, a planetary automatic transmission that can be shifted without traction force interruption, or a continuously variable transmission.
When a hybrid drive-train of this type is operating under electric power and a power increase is called for which can no longer be satisfied by the electric machine alone, the internal combustion engine has to be started and coupled to the input shaft in order, thereafter, to drive the motor vehicle under internal combustion engine power alone or together with the electric machine, i.e. in the hybrid driving mode, with a correspondingly increased traction force.
During accelerator pedal operation of the motor vehicle a higher power output of the drive aggregate, i.e. the electric machine and the internal combustion engine, is called for when the driver depresses the accelerator pedal farther, for example in order to accelerate the motor vehicle in order to overtake. When the vehicle is being operated with an activated speed regulation unit, a power output increase can be called for by the speed regulation unit, for example when driving onto an uphill stretch, in order to be able to maintain the specified desired speed.
A power output of the electric machine which is no longer sufficient for this can occur owing to a maximum torque of the electric machine predetermined by design and/or owing to an electrical energy accumulator which is no longer sufficiently charged. When a power increase is called for, to increase the traction force in particular from the internal combustion engine a downshift or increase of the transmission's gear ratio can also be carried out, and this gear ratio change can be carried out before, during or after the coupling of the internal combustion engine.
When a power increase is called for which triggers the switching on of the internal combustion engine, the internal combustion engine is first started and then, to enable the separator clutch to be closed with as little slip as possible and largely free from jerking, the internal combustion engine is accelerated approximately to the transmission input rotational speed before the separator clutch is substantially closed and the internal combustion engine thereby coupled to the transmission input shaft.
When the separator clutch is open the internal combustion engine can be started by means of a special starter which is permanently drive-connected to the driveshaft of the internal combustion engine. However, the internal combustion engine can also be drag-started in combination with the electric machine by partially closing the separator clutch, during which the electric machine preferably delivers a higher torque, and once a minimum starting rotational speed has been reached and the internal combustion engine has started, the separator clutch is opened again. Furthermore the internal combustion engine can also be impulse-started by the electric machine, but in this case the transmission has to be transiently opened for example by disengaging the currently loaded gear in an automated countershaft transmission or by opening a load-transmitting frictional shifting element of a planetary automatic transmission, and after the electric machine has accelerated the separator clutch is rapidly closed and then opened again once the internal combustion engine has started.
The acceleration of the internal combustion engine to the transmission input rotational speed, i.e. to the synchronous speed at the separator clutch, should take place as quickly as possible so that the drive torque increase usually demanded by the driver begins without significant time delay. When the acceleration of the internal combustion engine takes place automatically, i.e. by virtue of corresponding control or regulation of the operation of the internal combustion engine as such, to produce a high rotational speed gradient the torque produced by the internal combustion engine is first greatly increased and then, to avoid overshooting the target speed, sometimes reduced to zero.
Since the mass moment of inertia that results from the rapid acceleration of the internal combustion engine and acts upon its driveshaft is relatively large and difficult to control, the problem exists of how the internal combustion engine can be synchronized rapidly and as exactly as possible without overshooting the target rotational speed. On the other hand the load reduction of the internal combustion engine after or during the closing of the separator clutch should also take place as quickly as possible, but this conflicts with the sometimes necessary complete reduction of the torque produced in order to terminate the acceleration.
To overcome these problems, in methods for controlling a corresponding hybrid drive-train described in DE 198 14 402 C2 and DE 102 04 981 A1 it is in each case provided that the internal combustion engine is adjusted to the synchronous speed of the separator clutch, i.e. to the transmission input rotational speed, by means of a second electric machine such as a crankshaft-starter-generator connected to its driveshaft.
The method described in DE 198 14 402 C2 provides that by means of the electric machine, the internal combustion engine is accelerated in essence exactly to the current transmission input rotational speed and is then coupled to the transmission input shaft by closing the separator clutch. In this method, however, no account is taken of a possible change of the transmission input speed during the closing process of the separator clutch, caused by an acceleration or deceleration of the motor vehicle.
In contrast, the method according to DE 102 04 981 A1 provides that by means of the second electric machine the internal combustion engine is accelerated to a target speed higher than the current transmission input rotational speed, before the separator clutch is closed. The result is that during the closing of the separator clutch it can transmit a torque from the driveshaft of the internal combustion engine to the transmission input shaft, whereby a load change between overdrive operation and traction operation of the internal combustion engine, which is perceptible as an uncomfortable jerk, is avoided.
In contrast, DE 10 2007 050 774 A1 describes a method for controlling a corresponding hybrid drive-train, in which during rotational speed regulation the internal combustion engine accelerates automatically, i.e. without any assistance from an auxiliary drive, to the current transmission input rotational speed or to a speed higher than this by an offset value, before the separator clutch is closed to its gripping point and then, with farther closing of the separator clutch, the torque control is transferred from the electric machine to the internal combustion engine. From the torque variations in the average time diagram shown in FIG. 3 thereof it can be seen that the acceleration of the internal combustion engine to the transmission input speed and the closing of the separator clutch to its gripping point take a relatively long time, and that the torque produced by the internal combustion engine for acceleration is again reduced to zero in order to limit the speed increase, which delays and makes more difficult the subsequent load build-up of the internal combustion engine.